Vol fade (Not Sag)

[RockinRocket]

Encountering a weird phenomenon in my 68 model 1987 plexi build.

Vol drops (but comes right back and dips again) when wailing on E, D, C ext cords in a unusuall vol/EQ settings.

Bright Vol maxed.
Presance Maxed
Treble Maxed
Bass Zero
Middle Zero

If I bring in the mids or bass a bit it goes away with these same settings.
When using the Normal input this doesn't happen.
I have another 68 that is part for part the same except the one in question a different power transformer and has 30 more volts.

So what's going on here is the feedback loop acting up under full load of just top end frequancys?

[romberg]

Your description seems to match how blocking distortion is described:

http://www.aikenamps.com/index.php/what ... distortion

Merlin also covers this in his preamp book. The cure seems to be use smaller coupling caps and or larger grid stoppers. It might help to have a scope
to see exactly which stage is affected.

Mike

[Stevem]

What voltage are your coupling caps into the PI and out rated at?
They have a AC rating ( signal level) as well as D.C. , and there AC rating is half the D.C. Rating, or should be .
Some new caps fail in this regard!

[RockinRocket]

The caps are 400v nos mustards. Used them before with no leakage.

I was thinking it was a feedback loop